The influence of pH on the adhesive performance of phosphoric ester monomers with zirconia

Functional phosphoric ester monomers containing primers have been widely used in dental adhesives. This study aims to analyze the atomic-level interaction between zirconia and phosphoric ester monomers under different pH conditions (acidic or neutral).

Solid-state ³¹P NMR spectroscopy was employed to investigate the binding interactions between two types of phosphoric ester monomers-glycerophosphate-dimethacrylate (GPDM) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-with zirconia under varying pH conditions. Contact angles were measured on the zirconia surfaces with and without the application of phosphoric ester monomers. Additionally, tensile bond strength (TBS) tests were conducted to determine the effects of different phosphoric ester monomer-containing primers under different pH conditions on zirconia.

Solid-state ³¹P NMR spectroscopy revealed that both GPDM and MDP exhibited broadened peaks, positioned differently from their unreacted forms, indicating that the environment surrounding the phosphate groups in both GPDM and MDP changed upon adhesion to zirconia at the atomic level. Acidic MDP may form covalent bonds with zirconia through additional esterification on the zirconia surface. MDP-primed zirconia demonstrated a higher contact angle than GPDM-primed zirconia. The highest TBS was observed in the group of MDP reacted with zirconia under acidic conditions.

These findings suggest that phosphoric ester monomers, particularly MDP in acidic conditions, can chemically bind to zirconia, contributing to enhanced adhesion behavior.